potting material for high environmental pressure?

Hi, we have this subsea project where one of the challenges is to
install a circuit board on the outside of a retrievable subsea

installation (20" pipe), rated for 350 bar (5000 PSI) environmental
pressure, so the pressure load will be somewhat cyclic between 0 and 350
bar.
Some other equipment installed on this pipe is embedded in a hard,
polyurethane (PUR) potting. So I am thinking to also embed the circuit
board into this PUR potting. Since the surrounding potting material is
quite hard, ShA 80, and that it is significant differences in the
compressibility or bulk modulus of the PCB laminate (FR4) and the
potting material, we are concerned that the compression differences
between the PCB and the potting material will lead to high stress levels
on the components mounted on the PCB. The components on the PCB is a few
D2Packs and some 0603 resistors. The size of the PCB is 50x50 mm.
Two obvious alternatives is to put the PCB inside a 1-atm container. or
submerge the PCB in some pressure compensated oil container. The reason
not to choose one of these is to keep the size and complexity low.
So I am now thinking that this circuit board should first be potted in a
form of softer potting material before it is embedded in the harder PUR
potting surrounding the 20" pipe. So now I am wondering where to find,
or what kind of soft potting material to chose for this purpose? Anny
thoughts / experiences / suggestions, anyone?
Thanks..
Br,
Rune

Softer materials do have greater compressibility
due to their looser molecular structure, but the
difference is pretty small. I'd be way more
concerned about voids in your system. Like,
are there any bubbles between the packages and
the circuit board? Even within a plastic-packaged
device, there can be bubbles. At the least, I'd
pull a vacuum on potted assemblies before they
cure. Some volatiles can be expected from the
potting material, and these will contaminate the
chamber and pump. They are especially nasty
if they polymerize on contaminated surfaces,
which they probably will.
Shore A 80 polyurethane sounds like a pretty
good potting material to me. I don't see any
justification for going to a two-layer potting
system. Problems from incompatible materials
could make the cure worse than the disease.
For example, polyurethane may poison the catalyst
of a platinum-catalyzed addition-cure silicone.
This can cause uncured silicone in contact with
the cure inhibitor and the formation of hydrogen
gas bubbles. Don't want that! Simpler is safer!
Worries about stress from compressibility mismatch
should be addressed in the physical design, i.e.
cut slots in the PCB wherever you can. Anticipate
flexure and distortion of your assembly and design
stress relief such as S-shaped curves in conductors.

I think you're forgetting about the compressibility difference between
the potting compound and the FR4. That will very likely lead to serious
amounts of shear stress in the solder joints. Shore 80 polyurethane has
roughly the stiffness of a super ball.
I agree about making sure there are no voids under the parts, though I'd
be more worried about ceramic components than ICs.
Cheers
Phil Hobbs

That wound be a reason for using a flip-chip
underfill, which is designed to mitigate
solder ball stress, even if this isn't
flip-chip or BGA and there are no solder balls.
A stiff material like a flip-chip underfill
would also be protective for J-lead and
gull-wing connections.
A flip-chip epoxy underfill around the packages
and potting in polyurethane might be a good
solution.

Thanks, I am aware of those potential compatibility issues and that a
circuit could be adapted somehow to cope with the bulk modulus
differences. Due to other design constraints this circuit can not be
adapted this way so I am still in need of that two stage potting solution.
So far I have been suggested to try a inner potting of soft silicone
which I will run a pressure test on. If anyone have any other sugestions
I migth do a test on those to.
Br,
Rune

I think you might be better off going in
the other direction, like a Shore D20 or
D30 material. I believe Dexter offers
some epoxies in that range, intended to
be flowed under unpackaged chips used
for flip-chip on PCB.

Maybe, if that material has a bulk modulus equal or maybe grater than
that of the PCB, FR4, which is about 9GPa parallel to the PCB surface,
(according to some other tread I found, so might not be very accurate).
Do you have any examples of such materials?
Br,
Rune

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